The application of membranes in environmental protection

Desalination, 62 (1987) 149-164 Elsevier Science Publishers B.V., Amsterdam -

149 Printed in The Netherlands

The Application of Membranes in Environmental Protection* LIU TING-HUI Institute of Environmental Chemistry, Chinese Academy of Science, P.O. Box 934, Beijing (China) SUMMARY

In the last decade, our research has focused on four main areas: (1) Research into and preparation of membrane from polysulfone, polyvinylidene fluoride, sulfonated polysulfone, and a polyether sulfone with phenolphthalein lateral group. (2) The development of methods and apparatus for membrane research. ( 3 ) The design of ultrafiltration membrane modules and systems. ( 4 ) Applications of ultrafiltration. In future, we will carry on working in the same areas. We are especially interested in developing new membranes with better separation characteristics and improved thermal, chemical and mechanical properties.

INTRODUCTION

Since Loeb and Sourirajan [l] produced the first successful membrane for water desalination at the University of California in 1960, the study of membrane processes (hyperfiltration or reverse osmosis and ultrafiltration) has grown into a new field of applied chemistry and chemical engineering. Of all the membrane processes, ultrafiltration has the largest variety of applications. It is utilized in the treatment of process waters, for the concentration, purification and separation of macromolecular solutions in the chemical, food and drug industries, for the sterilization, clarification and purification of biological solutions and beverages, and for the production of ultra-pure water. In recent years, environmental protection has become one of the most important areas in which membrane technology, especially ultrafiltration systems, has been applied. Commercial ultrafiltration systems have been widely used to improve the treatment of indutrial effluent. At the Institute of Environmental Chemistry in Beijing, we have been study*Presented at the International Symposium on Synthetic Membrane Science and Technology, Dalian, China, April 13-18,1986. OOll-9164/87/$03.50

0 1987 Elsevier Science Publishers B.V.

150

ing membranes since 1975. In this paper, our research work on reverse osmosis and ultrafiltration membranes and their applications in environmental protection and the treatment of process waters is briefly described. RESEARCH AND PREPARATION OF MEMBRANES

Because our Institute is concerned with environmental science, we are mostly interested in membranes with high resistance to chemicals, pH, temperature, and microbial degradation and in their applications in industrial wastewater treatment. This is why membranes made from polysulfone, sulfonated polysulfone, polyvinylidene fluoride, and polyethersulfone with a phenolphthalein lateral group have been studied in our Institute. Of course, these kinds of membranes can also be used in other areas such as the chemical, food and drug industries. Preparation of polysulfone (Udel) UF membrane [2] Polysulfone is as follows:

C’-b

is a plastic developed

in the sixties whose molecular

structure

0

It is a good ultrafiltration membrane material, exhibiting excellent oxidation resistance, thermal stability and toughness. The effects of casting solution composition and casting variables on the performance of membranes were investigated. Three types of UF membrane, PSM10, PSM-60 and PSM-100 were manufactured. Their molecular weight cutoffs are 10,000,60,000 and 100,000 respectively. It was found that the polysulfone content in the casting solution is the most important factor influencing the performance of membranes, as can be seen in Fig. 1. Casting solutions with the additive polyvinylpyrrolidone or ethylene glycol methyl ether were used in this investigation. Solute retention of membranes for bovine serum albumin (BSA, mol.wt. = 67,000) and cu-chymotrypsin (CHT, mol.wt. = 27,500) increases and the flux of the membrane decreases with increase in concentration of polysulfone. It was found that the stack density of polymer matrix in the membrane increases, while the pore size and porosity on the membrane surface decrease with increase in content of polysulfone. The pore size distribution changes in different ways, depending on the additive used, with increase in polysulfone content. The effect of additives on the membranes was examined. DMF was used as

151

90

Polysulfone

content.

wt %

Fig. 1. Effect of polysulfone content in casting solution on the performance of membranes. Operating pressure 3 kg/cm*. 0 EGME BSA CHT 0 EGME + PVP BSA 0 PVP CHT

TABLE I EFFECT OF ADDITIVES ON MEMBRANE PERFORMANCE Additive

None Methyl ethyl ketone 8% EGME 8% PVP 8% PEG-400 8%

Pure water flux, ml/cm’ h 33 20 84 123 138

Retention, % CHT

BSA

78 84 83 78 76

91 98 93 92 89

152 TABLE II EFFECT OF EGME CONTENT ON MEMBRANE PERFORMANCE EGME content, wt%

Pure water flux, ml/cm’ h

Retetention,% CHT

BSA

0

33

18

-

4 8 12 16

30 a4 80 95

80 83 a4 80

91 93 90 92

TABLE III EFFECT OF PVP CONTENT IN CASTING SOLUTION ON MEMBRANE PERFORMANCE PVP content, wt% 4 8 12 16

Pure water flux, ml/cm’ h 122 123 133 129

Retention, % CHT

BSA

IO 78 75 60

94 92 95 97

the solvent and four kinds of additive were tested for flux and solute retention (Table I). We consider polyvinylpyrrolidone ( PVP) , ethylene glycol methyl ether (EGME) and polyethylene glycol-400 (PEG-400) to be suitable additives. The effects of adding different quantities of EGME and PVP are given in Tables II and III, respectively. The experimental data show that 8 wt.% content of EGME or 4 wt% of PVP is enough to ensure high permeation and retention. The gelation bath temperature is one of the main casting variables for making polysulfone membranes. Two kind of casting solution were examined. With water as the gelation medium, the gelation bath temperature affected membrane performance in different ways depending on which additive was used. As Fig. 2 shows, the flux increases with increase in gelation temperature, using either EGME or PVP as an additive. When EGME is added to either of the two solutes used, increasing the gelation temperature has practically no effect on retention. However, when PVP is used as an additive, the retention for the a-chymotrypsin solute decreases with increase in gelation temperature. It seem that in this case the pore size distribution of the membrane becomes much wider when the gelation temperature is increased.

153

Preparation of sulfonated polysulfone RO/UF membranes {3-51 In order to enhance the hydrophilicity of the membrane, a homophase sulfonation reaction of polysulfone was carried out using chlorosulfonic acid as a sulfonating agent in dichloroethane solution. The sulfonated polysulfone produced in this way was used to make RO membranes. A heterophase sulfonation reaction of polysulfone was carried out also. When chlorosulfonic acid is titrated into a mixture of polysulfone powder and an organic solvent, the sulfonating reaction occurs on the surface of the polysulfone powder. The product of this reaction was used to make UF membranes. The mechanisms of these two reactions may be expresed as follows:

The effects of the polymer and additive content in the casting solution, the temperature of the casting solution, the period of solvent evaporation, and the composition and temperature of the gelation medium on the performance of sulfonated polysulfone RO membranes were studied. The basic composition of the casting solution was: sulfonated polysulfone 25.0 (wt.%) dioxane and acetone 70.2 water 4.8 The resulting membranes were tested using 5,000 ppm NaCl aqueous solution at 25”C, under 40 kg/cm2 operating pressure. The product flux was 2-3 ml/cm*h and the rejection was 90-95%. Fig. 3 shows the relation between the characteristics of sulfonated polysulfone membranes and the polymer content of the casting solution. Initially, the product flux decreases rapidly and the rejection increases rapidly with increase

I

0

I

10 Temperature

I 20

of

I

I

I

30

40

50

gelation

bath,

“C

Fig. 2. Effect of gelation bath temperature on trhe performance of polysulfone membranes. For experimental conditions see Fig.1.

in polymer content. The reason for this is that when the period of solvent evaporation is short, the concentration of polymer in the top layer is the same as that in the sublayer of the liquid membrane. So in this case the membrane resulting from a low polymer content casting solution has an imperfect surface layer, while the membrane resulting from a high polymer content casting solution has a better skin layer. As the period of solvent evaporation becomes longer, the effect of the polymer content in the casting solution becomes weaker. This is because the period of solvent evaporation is long enough to form an almost perfect skin on the top layer of the membrane, no matter what the polymer content. We examined the resistance of sulfonated polysulfone membranes to chemicals by immersing them in various chemical solutions for weeks or months and tben testing their performance. The results are given in Table IV; they show that sulfonated polysulfone membranes have good resistance to chemicals.

155

01 21

’

’

Sulfonoted

’

’

25

’

polysulfone

’

’

content,

’

’

30

%

Fig.3. Effect of polymer content in casting solution on the performance pof sulfonated polysulfone membranes operating pressure 40 kg/cm’, 5000 ppm NaCl, 25’ C. Solvent evaporation time ( 0 ) 2s, (t-j lOs, (0) 15s.

The formation of UF membranes from sulfonated polysulfone produced by the heterophase sulfonation reaction was studied. The experimental data show that the ion exchange capacity (IEC) of the polymer has a strong effect on the charcteristics of the membranes. As the IEC of the polymer increases, the pure water flux also increases until the IEC reaches a certain value above which there is no further increase in the pure water flux. In the experimental range of IEC, the retention for cytochrome C (mol.wt. = 13,000) is above 90% and the pure water flux is 80-100 ml/cm2h under 3 kg/cm2 operating pressure. Preparation of polyvinylidene fluoride (PVFD)

membrane [S]

Polyvinylidene fluoride is a good UF membrane material with excellent chemical and thermal stability. The effects of the composition of the casting

156 TABLE IV RESISTANCE TO CHEMICALS OF SULFONATED POLYSULFONE Resistance to

Retention, %

Immersion medium

36 d

60 d

84 d

120 d

204 d

acid

0.1 N HCl

90.0

91.0

base

0.1 N NaOH 0.5 N NaOH

91.5 91.9

-

90.2 -

94.3

90.1

oxidant

5 g/l K&&O, + 5 g/l H,SO,

96.9

-

-

94.9

-

5 g/l CrO:s +0.5 g/l H,SO,

96.6

-

-

94.7

-

15 g/l CrOrl + 1.5 g/l H,SO,

88.9

91.4

-

-

-

TABLE V EFFECT OF PVFD PERFORMANCE

CONTENT

IN

CASTING

SOLUTION

PVFD content, %

Pure water flux, ml/cm’ b

Retention for BSA, %

16 17 18 19 20

28 24 24 5 4

90 92 90 91 94

ON

MEMBRANE

solution and the casting condition were investigated. The casting solution composition was polyvinylidene fluoride/MDF and acetone/PEG-600, H/72/10 (w/w). It was found that the pure water flux of PVFD membranes clearly decreases with increase in PVFD content in the casting solution, but the retention for BSA does not change a lot (Table V). This means that an increase in the polymer content mostly influences the porosity, but not the average pore size of the resulting membrane. The period of solvent evaporation directly affect the performance of the PVFD membrane. As Fig. 4 shows, the pure water flux passes through a maximum. PVFD membranes were immersed in 20 chemical for one month and then

157

0

I

10

20

I

I

I

I

40

60

60

la,

Solvent evaporation

I

1P

period, s

Fig. 4. Characteristics of PVFD membrane as a function of solvent evaporation period. UF test conditions: operating pressure 3 kg/cm*, feed solution 0.1% bovine serum albumin.

tested. The data in Table VI show that membrane performance was not greatly affected, except for those membranes immersed in ethyl acetate and nitric acid. Preparation of a new UF membrane from polyether sulfone with phenolphthalein lateral group (PDCI A novel type of polyether sulfone with phenolphthalein lateral group was synthesized by Liu Ke-jeng and Chang Hei-chun [ 71. The repeating unit of the PDC molecular structure is as follows:

There is a phkolphthalein

lateral group in the main chain of PDC, so it has

158 TABLE VI RESISTANCE

TO CHEMICALS

Immersion medium

5% NaCl 95% alcohol 1 NHCl 12 N HCl 1 N H,S04 65% HNO, 1 N NaOH 0.1% KMnO, 0.5% KMnO, Acetic acid Benzene Dimethylbenzene Monochlorobenzene Dichloroethane Chloroform Hexane Cyclohexane Acetone EGME Ethyl acetate

OF PVFD MEMBRANE

Before immersion

After immersion

Flux, ml/cm’ h

Retention, %

Flux, ml/cm’ h

Retention, %

28 30 10 10 12 8 16 10 10 8 15 16 11 11 14 35 40 10 18 8

86 91 96 95 95 95 92 90 95 93 94 94 95 95 95 94 90 93 95 91

20 28 8 8 12 8 17 7 7 8 14 15 10 10 12 30 30 8 15 2

85 88 83 86 86 75 84 83 88 89 89 89 88 87 85 85 88 89 87 57

higher hydrophilicity than Udel polysulfone. Experiments have shown that the pore size of the PDC membrane is also more easily adjusted, while the pore size distribution is quite uniform. THE DEVELOPMENT

OF METHOD

AND APPARATUS

FOR MEMBRANE

RESEARCH

In order to study in depth the charcteristics of membranes and the relationship between properties and structure, we have developed some physico-chemical method and the corresponding equipment in our laboratory. Measurement of solvent evaporation rate constant Solvent evaporation is an important step during L-S type membrane formation. The solvent evaporation rate is a function of the solution structure and temperature and the nature of the casting atmosphere. Kunst and Sourirajan [ 8 ] have investigated this step in detail. The linear part of the evaporation rate curve can be represented by the relation

159

(wt-W,)=(W,--W,)exp(-bt)

(1)

where W, = weight of casting plate film at any time, IV,,= value of W, at time = 0, W, = value of W, at the time when solvent loss has practically ceased. The evaporation rate constant b is a useful quantitative parameter to express the evaporation rate during film formation. An apparatus for determining the value of b was developed in our laboratory and used to study the relationship between the evaporation rate and the performance of polysulfone UF membranes [91. Observation of UF membrane structure Observation of the structure of the UF membrane sublayer by optical scope is a simple and effective method of studying membranes. The graphs provide some information about the effects of casting conditions microporous structure of the membrane sublayer, from which the pore ture of the skin layer can be guessed.

micromicroon the struc-

The study of membrane materials by means of high-performance liquid chromatography Matsuura, Sourirajan and others [ lo-121 have proposed that high-performance liquid chromatography ( HPLC ) be used to simulate the equilibrium conditions prevailing at membrane-solution interfaces in RO/UF experiments. HPLC data offer an effective means of characterizing membrane materials relevant to RO/UF applications. The necessary research methods and apparatus have been developed in our laboratory. UF MEMBRANE MODULES AND SYSTEMS DESIGN

For the successful application of ultrafiltration as an efficient mass separation process, the development of membrane systems and of different configurations for containing the membrane is very important. The main task in designing an UF module is to control concentration polarization effects and to increase the membrane area in relation to the module volume. Commercially available UF configurations include: plate an frame, tubular, hollow fiber or capillary, and spiral wound. Two of them were developed in our laboratory: the external pressure tubular and hollow fiber modules. The external pressure membrane module and system The external pressure module provides significant advantages in terms of tolerating a high concentration of suspended solids in the feed solution and

160

control of concentration polarization. The construction of the module is shown schematically in Fig. 5. Here the membrane is cast on the outside surface of an 80 cm long porous plastic tube with an outside diameter of 8 mm and an inside diameter of 3 mm. 85 of these membrane tubes are housed in a cartridge. The feed solution flows around the outside of the tubes and the permeate is collected inside them. The UF system consists of several of these cartridges installed in parallel array (Fig. 6). It has been successfully used in the recovery of colour film developing solution in the photoprocessing industry, which we will talk about later. Hollow fiber modules and systems The hollow fiber module is shown schematically in Fig. 7. Fibers are housed in a fiberglass-reinforced or ABS plastic cartridge. The feed solution flows t

Permeate

Rubber stopper

Concentrate

-Shell

Men-Wane

tube

Fig. 5. Construction of external pressure tubular module.

161

Fig. 6. External pressure tubular HF system. Resin

end

Hollow

fiber

Permeate

Fig. 7. Hollow fiber UF module.

through the inside of these fibers and the permeate is collected on the outside. Two different inside diameters (0.5 mm and 1.1 mm) of hollow fiber are available in our laboratory. The hollow fiber module may be installed in series or parallel array. The system provides good feed control and a large membrane

162

surface area per unit volume. It has been used to treat dyeing wastewater, oil wastewater, textile oil, and so on.

crude

APPLICATION OF ULTRAFILTRATION

Generally, ultrafiltration membrane applications can be divided into three areas: waste treatment, the treatment of process waters and pure water production. Our research work on the applications of ultrafiltration focuses on the first two areas. Recovery of colour film developing solution [13] During the process of developing colour films the concentration of bromide, oxide and gel in the developing solution gradually increases. In the old developing process, in order to maintain quality, it was necessary to continuously add fresh developing solution into the solution bath and to discharge the overflowing solution. This resulted in high photoprocessing costs and serious environmental pollution. Now, by combining the ultrafiltration and ion exchange processes, the waste developing solution can be treated, and if a little developing agent is added to the solution after it has been treated, it can be reused to develop the colour films again. A flow diagram of this system is shown in Fig. 8. This technique has already been widely applied in twenty photoprocessing factories in China. Reclamation of textile oil (spin finishes) In the Vinylon spinning process, spin finishes easier and to reduce electrostatic interaction. In used solution contain a certain quantity of spin charged as wastewater. In this way, about one

Q/

are used to make the spinning the wet-finishing process, the finishes, and it is usually dishundred tons of spin finishes

Fig. 8. Flow diagram for treatment of colour film developing solution.

163 were being lost per year in the production

of Vinylon in China. In 1983, we successfully concentrated this wastewater using a hollow fiber ultrafiltration system in the Beijing Vinylon Factory [ 141. The concentrate can be reused and the COD value of the effluent is lowered.

Dye reclamation in the printing and dyeing industry [15] After dyeing, the concentration of disperse dye in the dyeing solution decreases, and until recently the solution would not be used again. This effluent was polluting the environment as well as throwing away a valuable material the dye itself. The UF membrane system has now been utilized to concentrate overflow solution containing disperse dyes. The retention for dyes is over 94% and the concentrate can be reused. Using one set of model CLB-250 UF equipment (treatment capacity 250 l/h), the net annual saving to industry is 120,000 Chinese Yuan. Treatment of crude oil wastewater After long distance transportation of crude oil by oil line, large quantities of water containing 100-1000 ppm crude oil can become separated from the oil, causing a serious pollution problem. Although the separation of oil-water emulsions has been a commercially important application of ultrafiltration, the treatment of crude oil wastewater by UF has not yet been covered in the literature. After laboratory research, an industrial scale hollow fiber UF sys-

Ol

I

I

I

I

50

100

150

200 Operating

,

I

250

I

300 period

Fig. 9. Product flux as a function of operating period.

,h

350

1

I

4&I

450

X

164

tern with a treatment capacity of 2000 l/h was designed and installed in an oilline terminal in Beijing. After floating oil had been skimmed off, the wastewater was pumped through a filter and UF unit consisting of 9 modules with an outside diameter of 100 mm. The crude oil content in the permeate was less than 10 ppm. The membrane flux for 500 h of operation is shown in Fig. 9. The treatment capacity and the quality of the permeate fulfilled all design requirements. Some other applications of UF The recovery of water and polyvinyl alcohol from textile printing effluents by hollow fiber membrane has been studied in our laboratory [ 161, and pilot scale equipment is now operating in the Beijing Second Printing and Dyeing Mill. Another application of UF we are working on is the clarification of wine. As obtained from the fermentation process, wine contains small amounts of proteins, polysaccharide and other colloidal impurities. Ultrafiltration of wine removes these impurities without loss of wine fragrance. The final object of our research is the purification and concentration of enzymes, which will play an important role in biological engineering.

REFERENCES 1 2 3 4 5 6 7 8

9 10 11 12

13 14 15 16

University of California, Office of Public Information, Press Release, New Water Desalting Process Developed at UCLA, Aug. 23,196O. S.J. Han,T.-H. Liu,K.F. Wu,R.S.Xu,G.Y. WangandS.S.Li, J.Environ.Sci., l(6) (1980). Anon., A reverse osmosis material - Preparation of sulfonated polysulfone, Environ. Sci., 5(5) (1980). Y. Shen, B.H. Feng and Z.-Z. Liu, Desalination (China) 2 (1980) 77. Y. Shen, B.H. Feng and Z.-Z. Liu, J. Environ. Sci., 3 (1) (1982). S.-J. Liu, J.-R. Wang and F.-L. Liu, Polyvinylidene fluoride VF membrane, Membr. Sci. Technol., l(2) (1981) 15-23 (in Chinese). K.-J. Liu and H.-C. Chang, Plastic Industry, 4 (1) (1980). B. Kunst and S. Sourirajan, J. Appl. Polym. Sci., 14 (1970) 1983. Y.-B. Wang and B. Chang, Casting PS film and membrane performance, Membr. Sci. and Technol., 2 (2) (1982). T. Matsuura, Y. Taketani and S. Sourirajan, AF. Turbak (Ed.), Synthetic Membranes, Vol. II, ACS Symp. Ser. 154, American Chemical Society, Washington, D.C., 1981, pp. 315-338. T.-H. Liu, Use of high-performance liquid chromatography for the choice of polymeric membrane materials. Membr. Sci. Technol., 2(4) (1982) 1-12. T.-H. Liu, K.-H. Chan, T. Matsuura, and S. Sourirajan, I & EC Product Research Development, 23 (1984) 116. Appraisal of Science and Technology, Yi Zi 028, Chinese Academy of Sciences (80)) Application of UF in recovery of colour film developing solution. S.-J. Han, S.-S. Li, K.F. Wu, Y. Shen. S.G. Xu and F.L. Liu, Environmental Chemistry, 3 (4) (1984). Appraisal of Technology, Ke Zi 84101, Ministry of Textile Industry. T.-H. Liu, J.-R. Wang, G.-X. Wu, Y. Shen, S.G. Xu, Z.Z. Liu and F.L. Liu, Environ. Chem., 3(5) (1984).